A reflux condenser attracts attention from the standpoints of improvement in productivity by increase in cooling capacity as well as saving in energy, and recently, it is frequently employed in particularly large-sized polymerization reactors. But, the well-known cooling method has a problem that the overall heat-transfer coefficient of reflux condensers lowers with the progress of polymerization to result in the reduction of heat-removing capacity and reduction of the stability of polymerization temperature control.
In order to solve these problems, there have so far been proposed the following methods: A method to raise the heat-transfer capacity by forced vapor circulation into a reflux condenser (described in Japanese patent publication No. 29196/1976); a method to control the capacity by the weighted average of the readings of one or several temperature detecters set up in the reflux condenser (described in U.S. Pat. No. 4,061,848), and the like. But, the former method requires expensive circulating blowers, and the latter one requires large-sized condensers because of a small heat-transfer coefficient. Thus, both methods were not always said to be satisfactory.
In cooling methods with a reflux condenser, it is thought that the overall heat-transfer coefficient of the condenser is generally not less than 500 kcal/m.sup.2 .multidot.hr.multidot..degree.C. In practical polymerization, however, the coefficient was as low as 100 to 150 kcal/m.sup.2 .multidot.hr.multidot..degree.C., so that very uneconomical large-scale equipments were required.
The reduction of heat-transfer coefficient may be considered to be due to non-condensable gases in the reactor, for example air present from before the polymerization, and CO, CO.sub.2 and N.sub.2 generated by decomposition of a polymerization initiator, which are carried to the surface of heat-transfer tubes of the condenser together with condensable vapors or gases such as the monomer, etc., and concentrated thereat.